1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition suitable for the piezoelectric layers in various piezoelectric elements such as an actuator, a piezoelectric buzzer, a sound component and a sensor, and to a laminated piezoelectric element using the above-mentioned composition, and in particular, relates to a piezoelectric ceramic composition capable of being sintered at temperatures of 950° C. or lower and in a low-oxygen reductive atmosphere.
2. Description of the Related Art
Piezoelectric ceramic compositions to be used in such a piezoelectric element are required to be high in piezoelectric properties, in particular, large in piezoelectric strain constant. As piezoelectric ceramics capable of attaining a large piezoelectric strain constant, there have hitherto been known, for example, a ternary system (PZT) composed of lead titanate (PbTiO3; PT), lead zirconate (PbZrO3; PZ) and lead zincate niobate (Pb(Zn1/3Nb2/3)O3), or a piezoelectric ceramic in which the lead (Pb) in the above-mentioned ternary system is partially substituted with strontium (Sr), barium (Ba), calcium (Ca) or the like.
However, these conventional piezoelectric ceramics require high sintering temperatures of the order of 1200° C.; consequently, in the production of laminated piezoelectric elements, it is necessary to use, for internal electrodes, high-price noble metals such as platinum (Pt) and palladium (Pd), leading to a problem of increased production cost. Accordingly, for the purpose of using a lower-price silver-palladium (Ag—Pd) alloy for internal electrodes, the sintering temperature has been demanded to be decreased.
Under these circumstances, the present applicant has proposed in Patent Document 1 to enable low-temperature sintering at 1050° C. or lower by adding a first additive containing at least one selected from Fe, Co, Ni and Cu and a second additive containing at least one selected from Sb, Nb and Ta to the above-mentioned ternary piezoelectric ceramic compositions, and consequently to make usable low-price materials such as a Ag—Pd alloy for internal electrodes.
Recently, use of copper (Cu), lower in price than the Ag—Pd alloy, for internal electrodes has been investigated. The use of Cu, having a melting point of 1085° C., for internal electrodes requires the sintering temperature to be set at 1050° C. or lower. In this connection, it is to be noted that Cu starts to be sintered at a further lower temperature, so that the sintering temperature is required to be set at a temperature as low as possible, for example, at 950° or lower. Additionally, Cu is a base metal, and sintering of Cu in air leads to oxidation thereof to make Cu unusable as electrodes. Accordingly, fabrication of laminated piezoelectric elements using Cu for internal electrodes requires the sintering to be carried out in a low-oxygen reductive atmosphere.
In response to the above-described requirement, the present applicant has proposed in Patent Document 2 a production method of a piezoelectric ceramic in which Pb in a ratio of 0.01 to 1.5% by mass in terms of oxide (PbO) and Cu in a ratio of 1% by mass or less in terms of oxide (CuO) are added to a calcined powder having a composition represented by (Pba−bMeb)[(Zn1/3Nb2/3)xTiyZrz]O3 and then sintering is carried out, wherein a, b, x, y and z are the values respectively falling within the following ranges 0.96≦a≦1.03, 0≦b≦0.1, 0.05≦x≦0.40, 0.1≦y≦0.5 and 0.2≦z≦0.6 with the proviso that x+y+z=1, and Me represents at least one element selected from the group consisting of strontium (Sr), calcium (Ca) and barium (Na).
[Patent Document 1] Japanese Patent Laid-Open No. 2004-137106
[Patent Document 2] Japanese Patent Laid-Open No. 2006-193414